MEDIUM
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A black body is defined as a perfect absorber of radiations. It may or may not be a perfect emitter of radiations.

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Important Questions on Thermal Properties of Matter

EASY
If temperature of a black body increases from 300 K to 900 K, then the rate of energy radiation increases by
HARD
A human body has a surface area of approximately 1 m2. The normal body temperature is 10 K above the surrounding room temperature T0 . Take the room temperature to be T0=300 K . For  T0=300 K, the value of σT4=460 W m-2(where σ is the Stefan-Boltzmann constant). Which of the following options is/are correct?
HARD

The filament of a light bulb has surface area 64 mm2. The filament can be considered as a black body at temperature 2500 K emitting radiation like a point source when viewed from far. At night the light bulb is observed from a distance of 100 m. Assume the pupil of the eyes of the observer to be circular with radius 3 mm. Then:

(Take Stefan-Boltzmann constant =5.67×10-8 W m-2 K-4, Wiens' displacement constant =2.90×103 m K, Planck's constant =6.60×1034 J s, speed of light in vacuum =3.00×108 ms1)

MEDIUM
Two spheres S1 and S2 have same radii but temperatures T1 and T2 respectively. Their emissive power is same and emissivity is in the ratio 1:4 Then the ratio of T1 to T2 is
EASY
Two spherical black bodies have radii r1 and r 2. Their surface temperatures are T1 and T 2. If they radiate same power then r2r1 is
EASY
A black body has maximum wavelength λm at temperature 2200 K. Its corresponding wavelength at temperature 3300 K will be
HARD
The maximum wavelength of radiation emitted by a star is 289.8 nm. Then intensity of radiation for the star is (Given : Stefan’s constant = 5.67×10-8 W m-2 K-4 , Wien’s constant, b=2898 μm K)
MEDIUM
The power radiated by a black body is P and it radiates maximum energy at wavelength, λ0. If the temperature of the black body is now changed so that it radiates maximum energy at wavelength 34λ0, the power radiated by it becomes nP. The value of n is
MEDIUM
A spherical black body with a radius of 12 cm radiates 450 W power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be
HARD

A black coloured solid sphere of radius R and mass M is inside a cavity with a vacuum inside. The walls of the cavity are maintained at temperature T0. The initial temperature of the sphere is 3T0. If the specific heat of the material of the sphere varies as αT3 per unit mass with the temperature T of the sphere, where α is a constant, then the time taken for the sphere to cool down to temperature 2T0 will be

(σ is Stefan Boltzmann constant)

MEDIUM
Earth receives Sun's radiation at the rate of P W m-2. Mean distance between the Sun and the Earth is r m. Radius of the Sun is R m. If Stefan's constant is σ (in SI units), surface temperature of the Sun, in kelvin, is
MEDIUM
A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is U1, at wavelength 500  nm is U2 and that at 1000 nm is U3. Wien's constant, b=2.88×106 nm K. Which of the following is correct?
HARD
Three large identical plates are kept parallel to each other. The outer two plates are maintained at temperatures T and 2T, respectively. The temperature of the middle plate in steady state will be close to
EASY
Ordinary bodies A and B radiate maximum energy with wavelength difference 4 μm. The absolute temperature of body A is 3 times that of B. The wavelength at which body B radiates maximum energy is
EASY
Prevost's theory of heat exchange is not applicable at temperature
EASY
If temperature of black body increases from 17°C to 307°C, then the rate of radiation increases by
MEDIUM

Two black bodies A and B have equal surface areas and are maintained at temperatures 27°C and 177°C respectively. What will be the ratio of the thermal energy radiated per second by A to that by B?

HARD
Parallel rays of light of intensity I=912 Wm-2 are incident on a spherical black body kept in surroundings of temperature 300 K. Take Stefan-Boltzmann constant σ=5.7×10-8 Wm-2 K-4 and assume that the energy exchange with the surroundings is only through radiation. The final steady state temperature of the black body is close to
EASY
On observing light from three different stars P , Q and R , it was found that intensity of violet colour is maximum in the spectrum of P , the intensity of green colour is maximum in the spectrum of R and the intensity of red colour is maximum in the spectrum of Q . If Tp, TQ and TR are the respective absolute temperatures of P , Q and R , then it can be concluded from the above observations that :
HARD
What is perfectly black body? Explain Ferry’s black body.